[gdal-dev] Fema Flood Maps NAV88 - proj4text

[Chris English]

All,

I am trying to view some parcel maps (NJ state plane EPSG 102711) in conjunction
with FEMA Flood maps (EPSG 5703).  I can view one group, the compliment of FEMA
Flood maps, or the parcels, but not both, so clearly a transform is required.

I find myself looking for a proj4 text string for EPSG 5703 to (hopefully) transform in
PostGIS and view in qgis.  

I found this string >>proj=longlat +datum=WGS84 +no_defs +geoidgrids=g2009conus.gtx

in Ben Discoe's overview of Vertical and Geocentric coordinate support in OGR/Proj4
http://lists.maptools.org/pipermail/proj/2011-August/005844.html

and I've added g2009conus.gtx to my PROJ_LIB.

1) is the string the correct proj4 string for NAV88D

2) am I being hopelessly naive to think that just adding the string to a user-defined
    95703 in Postgis spatial_ref_sys is going to do anything useful to achieve the
    transform of the parcel maps so they may be viewed in conjunction with the
    FEMA maps

3) if 2 is correct, should I prepare the layer in gdal following Ben's approach
    and then import into Postgis

I am attempting this because when one is confronted with a flood map one wants
to know what is the elevation of one's particular parcel in relation the the flood maps
as against the more generalized representation of the flood zone stops about here.

thank you for your advice and consideration,

Chris English